The present invention relates generally to improvements to the method of construction of compacted granular or stone columns in soil masses.
Stone columns are columns of compacted granular material installed in soil for the purpose of improving the soil characteristics. Stone columns improve drainage in fine grained soil deposits and increase load bearing capacity to a point where considerably larger bearing stresses may be sustained without developing detrimental or excessive settlement or bearing capacity failure.
Stone columns, as the name implies, are vertical columns of compacted crushed stone, gravel or sand which extend through a deposit of soft material or soil to be strengthened. A number of acceptable methods are available for installing such compacted granular columns or stone columns. Examples of such prior art are disclosed in Mars U.S. Pat. No. 4,126,007 and in Goughnour U.S. Pat. No. 4,397,588.
Vibratory probes are typically utilized to construct a sand or stone column, the probe itself generally consisting of a 12 to 16 inch diameter hollow cylindrical body. A down-hole vibrator capable of providing a minimum of 130 kW and a minimum centrifugal force of 210 kN of force gyrating about a longitudinal axis is typically used to create lateral vibrations in the probe, i.e., vibrations in the horizontal plane. The minimum double amplitude (peak to peak measurement) of the probe tip is not less than 18 mm in a horizontal direction when the probe is in a free suspended position. In addition to the probe, there are follower tubes, electric cables, and/or hydraulic hoses and water hoses are connected usually to the uppermost extension tube. The complete assembly is usually supported from a commercial crane.
In order to construct a stone column with this type of probe, the probe is penetrated into the soil to a predetermined depth under its own weight and with vibration and assistance of a jetting fluid. The jetting fluid may be water under pressure or compressed air. At the required depth, the probe is slowly retrieved in 1 ft. to 2 ft. increments to allow backfill placement under the withdrawn tip of the probe. Granular material is transported to the probe tip through a transfer pipe that runs parallel to the probe down to the tip; in the industry, this is referred to as the bottom feed method. The transfer pipe is fed by a material container, typically mounted at the top of the probe assembly. As the probe is withdrawn, the granular material is fed into the void at the probe tip. The probe is partially lowered again, forcing the freshly deposited granular material into the surrounding soil with the assistance of vibration, thereby creating compaction. The ratio between the steps of extraction and re-driving governs the final cross section of the stone column. By repetition of these steps, the stone column is gradually constructed.
A problem that is frequently encountered in the current bottom feed method of stone column construction is the loss of continuous flow of granular material through the transfer pipe. The loss of flow has a number of detrimental aspects that are caused by the operator""s removal of the vibratory probe from the soil in order to determine if the flow is interrupted. In fully removing the vibratory probe, the construction of the stone column becomes time-inefficient. Another detrimental effect is that the construction of a uniform, well-compacted stone column is hindered.
The present invention overcomes the prior art deficiencies and improves the construction procedure by allowing the operator to continuously monitor the flow of material to the stone column thus guaranteeing a continuous stone column formation process in the most time-efficient manner. An object of the present invention is to provide the equipment operator real-time visual information on the flow of granular material conveyed from the material container into the stone column and monitor whether any granular material is left in the material container. Based on the visual information, the operator can control the stone column installation procedure, specifically the lifting and re-compaction phases without interruption in the flow of granular material.
This new system gives a qualitative assessment of the quantity of granular material remaining in the material container and the flow rate of stones conveyed through the transfer pipe and into the stone column. There are several advantages to real-time visual monitoring. The following are illustrative, but not exhaustive.
First, the operator knows if stones are actually flowing and does not need to lift the probe completely out of the ground for examination; this provides an efficiency gain in time and a reduction in effort. Second, the operator can control the rate of flow of granular material. Because stone placement varies according to soil response (soil response depends on such disparate factors as fines content, plasticity, water table level, etc.), an operator can continuously control and adjust the placement procedure by, for example, adapting the lifting speed, lift increment, or number of increments. Finally, as the stone column should be continuous and of only the minimum diameter specified, the method enables the operator to more efficiently reach this objective. A roughly 20% increase in average productivity over the construction of 200 columns has been measured.
Other objects and advantages appear in the following description and claims.